The City Council is set to tackle skyrocketing school bus costs at a hearing today in a bid to explain why the number of routes has ballooned over the past decade — even though ridership plunged dramatically.

Since 2000, the number of school-age students getting yellow bus service plummeted by nearly 30,000 — to 139,000 last year.

Yet the number of routes jumped from 5,400 to more than 6,700 — nearly doubling overall yellow bus costs to a whopping $1 billion.

The many factors that have driven up yellow bus costs include:

• The Department of Education added nearly 300 routes for summer school busing in 2000 and 700 routes for a new extended school day in 2005

• Bus companies have gotten an average payout increase of 3.1 percent each year since 2001 — plus an additional 5.5 percent bump in 2005